Commercial greenhouses are high tech production facilities for vegetables and flowers. Heretofore, such greenhouses have been adapted for use in regions with cold winters and arid summers. In such climates, the greenhouse interior may be heated in winter months such as by using boilers, heat pumps, or heat transferred from hot engine exhaust. Such greenhouses may be cooled during hot summer months through evaporative cooling systems. One example of such a cooling system is a pad and fan, which draws dry outside air through a wet porous pad to cool the air via evaporative cooling. To work, the fan must continuously draw ambient air through the pad and exhaust the cooled air to the outside. Fogging and misting systems are also widely used to cool greenhouses in arid environments.
In warm humid environments, cooling systems that depend upon evaporation for cooling are marginally effective at best. When considering water evaporating into air, the wet-bulb temperature which takes both temperature and humidity into account is a measure of the potential for evaporative cooling. The amount of heat transfer depends on the evaporation rate, which depends on the temperature and humidity of the air. As humidity increases, the actual air temperature approaches the wet bulb temperature. The less the difference between the wet bulb and actual air temperature, the less the evaporative cooling effect. Thus, in hot humid climates, the evaporative cooling effect is typically insufficient to maintain a greenhouse at a temperature favorable to plant growth.
Photosynthesis depends on a series of external and internal factors. The internal factors are the characteristics of the leaf (structure, chlorophyll content), the accumulation of products assimilated in the chloroplasts of the leaves, the availability of water, mineral nutrients and enzymes, among others. Among the most relevant external factors are the radiation incident on the leaves (quantity and quality), temperature, the ambient humidity and the concentration of CO2 and oxygen in the surrounding air. Excessive heat and humidity, or insufficient CO2, are non-limiting examples of factors that may stifle photosynthesis.
The concentration of CO2 in ambient outside air commonly varies from 300 to 500 parts ppm or more by volume depending on the season, time of day and the proximity of CO2 producers such as combustion or composting, or CO2 absorbers such as plants or bodies of water. Plants growing in greenhouses, particularly sealed structures, can reduce CO2 levels to well below ambient levels, greatly reducing the rate of photosynthesis. Conversely, enriching the concentration of CO2 above ambient levels can significantly increase the rate of photosynthesis. Consequently, many commercial greenhouses include CO2 enrichment systems to augment photosynthesis. Such systems typically supply CO2 from storage vessels or as a product of combustion of carbon-based fuel. The ideal concentration depends on the crop, light intensity, temperature and the stage of crop growth. However, 1000 to 1200 ppm is considered effective for many flowers and vegetables, with some exceptions. At this level, worker exposure should fall far below the 5,000 ppm permissible exposure limit for an 8-hour shift (measured as a time weighted average), as set by the U.S. Occupational Safety & Health Administration (OSHA).
In greenhouses with fan and pad evaporative cooling systems, it is difficult to maintain an elevated CO2 concentration, notwithstanding enrichment efforts. Such greenhouses continuously vent injected CO2 to the atmosphere. A vigorous stream of air must be maintained to provide cooling. The stream captures injected CO2. Venting releases it to the atmosphere with the air stream. This attenuates any benefit to photosynthesis while increasing production costs and compromising the outside environment.
An improved greenhouse climate control system that is capable of controlling temperature, providing heating and cooling as desired, and providing CO2 enrichment for optimal plant growth, in all ambient environments, including hot, cold, arid and humid, is needed. The invention is directed to overcoming one or more of the problems and solving one or more of the needs as set forth above.